DIYbanter - View Single Post

SMS

wrote:

More than likely, all you need are the green and red wires. The black
and yellow wires would be used for a second line if you had one. In a
normal residential system, they are not used, even though they may be
hooked to terminals as if they are.

In 1960's era houses they often used three conductor cable, and the
yellow wire was for lighted phone dials, via a transformer. Then they
went to four wire cable, with the red and green for the primary line,
and black and yellow for the secondary line. Often they'd install
multiple RJ11 jacks with only the inner pair used for each line, but
often they'd hook the black and yellow to the outer contacts, and the
red and green to the inner contacts.

You can often find two line phones that connect with a single cable,
pinned out that way.